This invention relates, in a first aspect, to a can or similar container having a can body the wall of which is internally lined with a protective coating, and a top opening therein, a membrane deep drawn from an aluminum foil or similar sheet material, serving as a warranty seal, which membrane comprises a flat part across the top opening and a collar part welded or glued sealingly to the coating on the inside wall of the can body, surrounding the top opening, which collar part extends from the plane of the flat part toward the rim of the top opening, with the upper edge of the flat part terminating on the inside of the can rim about the top opening, while a rim-covering part of the same foil or similar sheet material is crimped about the said can rim and extends on the inside wall, about the top opening of the can body, and terminates with its lowermost inner edge above the upper ede of the collar part of the membrane.
The invention also relates, to a process for sealing an opening of a can having a can body the wall of which is internally lined with a protective coating, which process comprises.
(a) introducing into the can opening a deep-drawn closure membrane having a flat part destined for covering the can opening, a collar part destined to be sealingly affixed on the inside of the wall of the can body near the can opening, and a peripheral part surrounding the collar part and destined for covering the rim of the can wall surrounding the can opening,
(b) crimping the peripheral part of the membrane about the rim of the can wall about the can opening thereby bringing the collar part and the adjacent circumferential zone of the peripheral part of the membrane to lie against the inside of the can wall adjacent and about the can opening in a contact zone of the membrane. In another aspect, the invention also relates to apparatus for carrying out the above-described process in practice.
Furthermore, the invention relates to a pre-formed closure element adapted for producing therefrom a can as described hereinbefore, which element comprises a membrane deep-drawn from an aluminum foil or a similar sheet material, which membrane consists essentially of a flat part, adapted for covering a top opening in the can body, and of a collar part destined to be glued or welded to the inside wall surrounding the top opening of the can body, which collar part has, in at least one annular membrance zone, parallel to the junction of the flat part and the collar part, a reduced thickness or a series of perforations, constituting a desired rupturing line; and the invention also relates to a process and apparatus for manufacturing a rupturing zone in a closure membrane destined for sealingly closing the opening of a can, and, in particular, the type of sealing closure to be found in the sealed can of the first described aspect of the invention.
It is conventional, especially in the case of cans containing foodstuffs, to not merely close the can with a lid, but also to provide, under the lid, a closure element consisting of a membrane of aluminum or aluminum laminate which can be pulled out of the can opening by tearing along a rupturing line or zone, after the lid has been lifted off the can.
Such aluminum membranes or foils are usually attached a few millimeters below the uppermost can rim and are provided by deep drawing with a marginal or collar part which extends in contact with the can wall upward to the said can rim. A tab is usually provided on the membrane, in particular on the flat part thereof which covers the can opening, and when pulling at the tab the flat part of the membrane will be torn out with a more or less clean tear which forms along a rupturing line or zone provided in the membrane.
This rupturing line or zone can be provided in the flat part of the membrane, or in the angle zone in which the flat part and the collar part join one another, or it can be provided in the peripheral marginal or collar part of the membrane.
In the case of round cans having a cylinrical body, the rupturing line or zone is usually a circular groove in which the thickness of the membrane is reduced.
It is known to produce this circular groove mechanically by scoring, which, however, has the drawback of reducing the thickness of the respective zone of membrane in a very irregular manner.
The wedge-shaped cross-section of the groove which occurs when the latter is produced by scoring with a conventional scoring tool produces centers of high tensile stress in the material which can lead to undesired rupture of the membrane due to jolting of the cans during transport.
According to another known method, a groove, to serve as rupturing zone, is crimped or impressed in the collar part of the membrane, which extends upward on the inside can wall toward the rim about the can opening. However, the reduction in the thickness of the membrane in this case is so small that, when the flat part of the membrane is to be severed and pulled out of the collar part, the membrane usually tears irregularly adjacent, but not along the desired rupturing line.
In a known device, which is inserted in a can, a membrane is permanently deformed between two pressure faces which are harder than the membrane, but only in a rupturing zone and, in particular, with a deformation which leads to a complete separation of the membrane into two parts.
A deformation of the membrane only in the rupturing zone between two hard pressure faces occurs, for instance, in the can closing method described in German Pat. No. 2,061,497 to Zeiler AG. In this can closing method the collar part of a deep-drawn membrane is glued onto the body of the can and is then cut through, whereby two separate membrane parts are formed which have along the incision line blunt edges abutting with one another.
This method has the drawback that it is practically impossible to make the incision with such precise depth that only the membrane and not the inside coating of the can, which is present in most modern cans, is cut; in practice, the incision will always penetrate through the coating into the material of the wall body itself. Regardless of whether this material is tin plate or cardboard, the destructin of the coating in the rupturing zone is highly undesirable for hygienic reasons, since, in particular if the can wall is made of cardboard, residues of a liquid content of the can will penetrate into the cardboard and will form crusts upon drying and become decomposed. In the case of metal cans, cutting of the coating, especially when the latter is of another metal, may cause electrochemical processes which may change the taste of the can contents. Moreover, cutting through the coating and into the material of the can wall may sever small particles thereof which may drop into the contents of the can.
The known methods have the common drawback that application of the desired rupturing line is time-consuming and demands a high accuracy of work. A further drawback consists in that, in the case of membranes of rectangular cans, it is difficult to produce a groove in the vicinity of the four corners of the can.